Caption: Heart valve replacement surgery. Surgeons' hands holding an artificial valve (centre) and attaching wires during an operation to replace a patient's defective heart valve. The heart valves prevent backflow of blood in the heart. If they start to fail, a condition known as incompetence, then the patient may not get enough oxygenated blood, leading to shortness of breath and chest pain. The condition may cause a heart attack in some cases. Replacement valves can either be artificial, as here, or from donors or even from other animals, usually a pig. During a valve replacement, the heart is stopped and its function taken over by a heart-lung machine.